Some members of the Polyomaviridae induce tumors in laboratory animals or in their natural hosts. In these cases, tumorigenesis in animals or transformation in cell culture requires the expression of one or more viral proteins called tumor antigens. The monkey virus SV40 and its close relatives BKV and JCV encode a large tumor antigen (LT) that acts as the main oncoprotein. Transformation and tumorigenesis induced by SV40 has been studied extensively and requires the interaction of LT with three types of cellular proteins: (1) the Rb-proteins (pRb, p107, and p130), hsc70, and p53. The association of SV40 LT with Rb-proteins, coupled with the action of hsc70, triggers E2F-dependent transcription of cell cycle genes and this action is sufficient to drive quiescent cells to proliferate. SV40 LT als binds to the DNA-binding domain of the cellular tumor suppressor p53 and thus inhibits its ability to bind and activate the promoters of p53-responsive genes. Consequently, the expression of cell-cycle arrest or cell death-inducing proteins is blocked. Like SV40, the LTs encoded by the human polyomaviruses BKV and JCV also bind and inhibit Rb-proteins and p53. However, these LTs induce transformation much less efficiently than SV40 LT. Several studies have suggested that the weaker transformation seen by BKV and JCV is due to lower transcription from the viral promoters. We have compared the transforming abilities of the SV40, BKV, and JCV LTs by driving their expression from the same common heterologous promoter. While BKV and JCV LTs accumulated to much lower steady state levels than SV40 LT, all three LTs were equally capable of disrupting the pRb pathway and inducing cell proliferation. However, BKV and JCV LTs transformed much less efficiently than SV40 LT in a number of assays. The amount of p53 - presumably bound and inactivated by each LT- varied considerably in each case and correlated to the amount of each specific LT present. We thus hypothesize that the increased transformation induced by SV40 is explained by a gain-of-function attributed to LT-p53 complexes. We predict that LT does not simply block the expression of p53-dependent genes - preventing p53 association with promoters - but that it also uses the p53 activation domain to stimulate the transcription of cellular genes thus contributing positively to transformation. We will test this hypothesis by a series of p53 ChIP-seq experiments. We also found that SV40 LT increases and controls the translation of the regulators CBP/p300, a function previously unknown to LT molecules. Because all SV40, BKV and JCV LT share a high degree of homology, we hypothesize that translational control is a common property of polyoma LTs. We will test this hypothesis by performing RNA-seq experiments on ribosome associated mRNAs. These studies will fill important gaps in our understanding of polyomavirus-induced tumorigenesis.